1. Field of the Invention
The present invention relates to driving circuits, and particularly to a driving circuit for providing a constant current.
2. Description of Related Art
Many electronic devices use driving circuits to provide constant currents. Portable electronic devices with display panels need a light source or sources to display pictures and other characters on the display. A matrix of light-emitting diodes (LEDs) are widely used as the light sources for these displays. Each LED in the matrix emits light by passing a current through a positive end and a negative end of the diode. A power of the light, influencing a brightness of the display, is directly proportional to the forward current. The driver circuit keeps the voltage and current constant so that the power of the LED light is constant.
Referring to FIG. 4, a traditional constant voltage driving circuit 100 includes a constant voltage source 110, a resistor 120, and an LED 130. The constant voltage source 110, the resistor 120, and the LED 130 are electrically connected in series. An anode of the constant voltage source 110 is electrically connected to the resistor 120, and a cathode of the constant voltage source 110 is electrically connected to ground. A positive end of the LED 130 is electrically connected to the resistor 120, and a negative end of the LED 130 is electrically connected to ground. Herein, a voltage of the constant voltage source 110 is labeled VBB, and a voltage of the LED 130 is labeled VF1, and a resistance of the resistor 120 is labeled R1. A forward current of the LED 130 is indicated with IF1. Thus, IF1 can be expressed in the following formula 1.IF1=(VBB−VF1)/R1   1
The LED 130 actuated by the constant voltage source 110 emits light with a constant power. However, when the LED 130 is in use for a long time, a temperature of the LED 130 increases and so does VF1. Therefore, according to formula 1, IF1 decreases, and a change of the power of the light occurs.
Moreover, VF1 often changes due to other factors such as signal noise. Once VF1 changes, IF1 also changes. In general, when VF1 changes 11%, IF1 changes 30%. Amplitude of the change of IF1 cannot be acceptable in practice.
Commonly, voltages of other loads often change as well as the LEDs while in use, making currents of the loads unstable. Therefore, the voltage driving circuit 100 cannot provide a constant current.
Therefore, a driving circuit is needed in the industry to address the aforementioned deficiencies and inadequacies.